1. Field of the Invention
The present idea relates in general to the field of retroreflected light and more particularly to an apparatus used to measure retroreflected light.
2. Description of the Related Art
Retroreflectance is the ability of an object or material to return light in the direction from which it came. This direction is defined by a narrow cone with its point on the object under test and opening toward the source, with a cone angle of less than a degree. Most materials retroreflect at least a small amount of light. For instance, white paint reflects (scatters) light in all directions, so some of the light must go back toward the light source. The amount of light retroreflected by white paint however is small because the light is almost uniformly scattered into a hemisphere (i.e. white paint is roughly Lambertian) so a cone with an angle of less than a degree would contain very little of the reflected light. A mirror is an example of an object that does not retroreflect, except when the light is normally incident. For all other angles of incidence, the light is reflected in a direction that has the same angle to the normal to the mirror surface but on the opposite side of the normal. A cube corner is a common example of a retroreflector; it retroreflects almost all of the light incident on it, although the return beam tends to be slightly displaced from the incident beam. A second example of a retroreflector is a glass sphere with a refractive index of 2.0, the back side of which is coated with a reflective material such as silver or aluminum.
Objects that exhibit retroreflectance are known in the art, and are commonly used in road signs. For example, U.S. Pat. No. 5,734,501, the disclosure of which is hereby incorporated herein by reference in its entirety, discloses a “Highly Canted Retroreflective Cube Corner Article.”
Further, ASTM International has a series of standards for measuring retroreflectance of materials used for road signs. These include D 4956 “Standard Specification for Retroreflective Sheeting for Traffic Control”, E 808 “Practice for Describing Retroreflection”, E 809 “Standard Practice for Measuring Photometric Characteristics of Retroreflectors”, E 810 “Test Method for Coefficient of Retroreflection of Retroreflective Sheeting Utilizing the Coplanar Geometry”, E 811 “Practice for Measuring Colorimetric Characteristics of Retroreflectors Under Nighttime Conditions”, E 1709 “Standard Test Method for Measurement of Retroreflective Signs Using a Portable Retroreflectometer at a 0.2 Degree Observation Angle” and E 2540 “Standard Test Method for Measurement of Retroreflective Signs Using a Portable Retroreflectometer at a 0.5 Degree Observation Angle”, the full disclosures of each of which are hereby incorporated herein by reference. These standards describe test methods and practices used for measuring retroreflective materials, but they do not completely specify the instruments used for performing the measurements. For example, E 811, Section 8.4 specifies a Light Projector Source that conforms to CIE Standard Source A (which is further defined in Section 3.2.5 as a “gas-filled tungsten-filament lamp operated at a correlated color temperature of 2855.6K) that produces a uniform patch of light on the sample and has either an adjustable iris diaphragm or a selection of fixed apertures. This leaves considerable discretion to the designer of the instrument regarding how to make the light patch uniform and obtain sufficient brightness on the sample.
Instruments for measuring retroreflectivity, sometimes referred to as retroreflectometers, are known in the art. Two broad categories of such instruments are those that include a beamsplitter, for example U.S. Pat. No. 4,368,982, the disclosure of which is hereby incorporated in its entirety by reference, and those that do not, for example U.S. Pat. No. 7,298,487, the disclosure of which is also hereby incorporated by reference in its entirety. In the latter case, the light source and the receiver are slightly displaced so that they do not occupy the same space. However, the relevant ASTM standards require that the angular separation of source and receiver be less than a degree, so the apparatus is inherently large due to the physical size of available light sources and receivers.
Retroreflectometers that include a beamsplitter work as follows. Light from a light source is reflected by the beamsplitter in the direction of the object under test. A portion of the light is also transmitted by the beamsplitter, but it is not relevant to the measurement and will be ignored. The reflected light impinges on the object and is, to some extent, retroreflected. The retroreflected light passes through the beamsplitter (again ignoring the irrelevant light) and is incident on the receiver. Because of the presence of the beamsplitter, the light source and receiver, although physically separated, may be optically coincident. This enables an apparatus that is more compact than one that does not have a beamsplitter. The tradeoff for use of a beamsplitter is that roughly three fourths of the light is lost.
One problem with the prior art instruments is that they either depend on a laser for illumination, see, e.g., U.S. Pat. No. 4,171,910, and thus measure retroreflectance at only one wavelength, or they depend on color filtration of the light. The two principle types of color filtration are photopic, as described in ASTM E 809-08, Section 8.2.1, and tristimulus, as described in ASTM E 811, Section 8.2.1. Photopic color filtration is intended to enable correction of the wavelength sensitivity of detectors that do not have a spectral response curve that mimics the human eye, thereby resulting in an electronic output that simulates the eye. Photopic filtration of detectors is difficult due to the significant difference between the spectral response curve of detectors and that of the human eye and because there are slight but significant variations from one detector to the next, even of the same type.
Tristimulus color filtration is used in instruments designed to measure color as well as optical power. Three filters and three detectors, which may be physically combined into one unit, are required. In general, one detector provides an electronic signal corresponding to blue input, one to green and one to red. The difficulty in creating accurate tristimulus filters that cause detectors to respond appropriately to light of different wavelengths is similar to the difficulty of creating an accurate photopic filter.
A second problem with the prior art instruments is that they assume that the distribution of retroreflected light is generally rotationally symmetric. In ASTM E 808-01, Section 6.1.2 and FIG. 5, an Intrinsic System of geometry is described, in which the receiver is positioned at an angle of gamma about the illumination axis to make a measurement. Unless gamma is variable in an instrument and a series of measurements is taken, a lack of symmetry in the distribution of the retroreflected light could produce a measurement error. This was not a significant problem with the older materials based on spherical glass beads, but the newer prismatic materials can create measurably asymmetric distributions. To circumvent this difficulty, ASTM E 1709 and E 2540 mention the use of a receiver that has an annular opening. While this allows simultaneous measurement of all values of gamma, it poses difficulties in the design of the receiver.
The present invention overcomes, inter alia, these difficulties.